The invention relates generally to thermal mass flow sensors and in particular to a tubular vertical flow thermal mass flow sensor for a vertical flow thermal mass flow meter or thermal mass flow controller. The tubular vertical flow thermal mass flow sensor is compensated for convection induced errors which are particularly prevalent with vertically oriented conventional thermal mass flow meters at zero and low flow rates.
In the manufacture of integrated circuits, it is necessary to perform multiple process steps including epitaxial growth steps, vapor deposition steps, diffusion steps and etching steps. All of these process steps are dependent upon chemical reactions with a silicon wafer in a reaction chamber or chemical reactions between or among the process gases in the reaction chamber. For instance, dichlorosilane and trichlorosilane are used in the epitaxial growth of silicon upon a substrate. If silicon is to be doped, phosphorus oxychloride, diborane, arsine and/or phosphine may be employed as dopant carrying gases in combination with silane compounds. Oxygen may be metered into a heated reaction chamber to cause thermal oxidation of silicon to take place to form silicon oxide. Silicon nitride may be deposited upon a silicon wafer by the reaction of ammonia and dichlorosilane. Wafers may be etched by gases such as sulphur hexafluoride excited in plasma reactors. The thickness and electrical characteristics of the substances deposited or grown on the wafer are influenced in part by the amount of reactant gas within the reactor. In order to control the amount of reactant gas in the reactor, mass flow controllers are connected between gas sources and the reactor to meter the flow of gases into the reactor to insure that the semiconductor manufacturing processes are performed properly. Failure to meter the gases properly may result in defective integrated circuits which must be scrapped. Mass flow controllers also may be used to meter anesthetic gases for use in medicine and for other precision process and analysis operations.
In the generally accepted terminology of the flow measurement industry a flow meter is an instrument for measuring the rate of flow of a gas, and a flow controller is a flow meter with a control valve and a feedback circuit combined to not only measure the flow but regulate it to a value that can be electronically set or manipulated. A flow sensor is a transducing element used within a flow meter or flow controller. It produces an electrical signal indicative of the rate of flow of a flow of fluid such as gas flowing through the sensor.
Specifically such sensors include a small diameter tube with a centrally heated region and means for sensing the temperature of the heated region at a pair of points along the tube. A flow of fluid such as gas through the heated region lowers the temperature of the upstream portion of the heated region and raises the temperature of the downstream portion of the heated region. The temperature differential results in a change in the signal, typically from a bridge including windings positioned about the heated region of the tube. A problem with such sensors is that the gas within the tube is heated which may give rise to gas convection currents within the tube.
Typically only a fraction of the total fluid flowing through the flow meter passes through the tube. The remainder passes through a path called a bypass which restricts the flow of gas so that the mass flow rate of gas through the bypass has a known relationship to the mass flow rate of gas through the tube. Usually the bypass flow rate is a linear multiple of the flow rate through the tube.
The heated tube and the relatively cool bypass form a loop in which convective flow or circulation may be established even when the valve of a mass flow controller is closed. Gas may convectively flow out one end of the tube into the bypass where it cools and back into the other end of the tube where it is again heated by the winding to continue the process.
Many times the mass flow controllers are incorporated in gas shelves as part of other processing equipment such as diffusion furnaces, chemical vapor deposition equipment, sputtering equipment, plasma etchers and the like. Most mass flow controllers are mounted so that the primary gas flow path through their bypass conduits is in a substantially horizontal direction. Typical mass flow controllers include a U-shaped sensor tube having a pair of vertically oriented legs connected by a substantially horizontal leg about which a temperature sensitive heater/sensor wire is wound. The heater/sensor wire comprises a portion of an electrical bridge which is excited from a source of electrical energy and whose voltage at a tap changes as the flow of gas through the sensor tube preferentially cools the upstream portion of the sensor winding with respect to the downstream portion of the sensor winding causing a voltage shift therein. It may be appreciated that at zero flow rates, the sensor windings will heat the gas within the sensor tube, but not cause any net convection of the gas in one direction, thereby avoiding convective flow through the U-tube.
Those skilled in the art have also attempted to use these conventional mass flow meters and controllers in applications where the flow through the mass flow meter, in particular through the bypass portion, is substantially in a vertical direction, thereby causing the sensor portion of the sensor tube also to be vertically oriented. The sensor portion is connected to the opposite ends of the bypass by an inlet leg and an outlet leg. At zero flow conditions when the heater-sensor winding is energized transferring heat to the gas in the sensor tube, the density of the gas is reduced relative to the density of the gas in the inlet and outlet legs causing the less dense gas to exit the upper portion of the sensor tube and drawing in denser gas at the bottom, causing the sensor tube to act as a chimney thereby creating a convective flow causing a shift in the sensed bridge voltage when in fact there was no net flow through the assembly. This results in a flow measurement error which is particularly significant when the flow rate is zero, and leads to a corresponding error at any flow rate.
Attempts have been made in the past to avoid this problem, for instance, with the apparatus disclosed in U.S. Pat. No. 4,056,975 to LeMay for Mass Flow Sensor System. The mass flow sensor system is difficult to manufacture in practice. One of the problems associated with the LeMay system is that the sensing wire comprising the upstream sensor 17 and the downstream sensor 18 is wound around a curved portion of the sensing tube 12. It is relatively difficult to obtain repeatable thermal characteristics in manufacturing sensors where the windings are placed around a straight section of the tubes, it is even more difficult to do so around curved sections. Thus, it has been found in practice that the LeMay system is impractical for reducing the convection effects on a commercial basis.
What is needed is a mass flow controller having a mass flow meter with a tube type sensor which may be oriented with its bypass substantially vertical and with the inlet to the system oriented either to cause the flow through the bypass to flow downward or flow upward and which is resistant to convective effects.